1. Field of the Invention
The present invention relates to a ceramic heater that can be used to help start diesel engines, wherein the heater includes a ceramic heating member.
2. Description of the Related Art
FIG. 7A shows a conventionally known ceramic heater 100, which can aid in the starting of a diesel engine. As shown in FIG. 7A, the conventional ceramic heater 100 includes a metallic cylindrical member 101 and a ceramic heating element 102, which is held at an end portion of the cylindrical member 101. The ceramic heating member 102 includes an insulating ceramic body 103 having a bar shape; a conductive ceramic element 104 having a U-shape, which is embedded in an end portion of the insulating ceramic body 103; and electrodes 105, which are embedded and thus connected to the respective end portions of the conductive ceramic element 104. Upon being supplied with electricity by means of the electrodes 105, the conductive ceramic element 104 generates heat through electrical resistance.
In the above-described ceramic heater 100, the cylindrical member 101 repeatedly expands and contracts from the heat generated by resistive heating of the ceramic heating element 102 as well as from repeated heating and cooling during combustion of the engine. As a result, a compressive stress is repeatedly exerted on the ceramic heating element 102. This compressive stress tends to become excessively large at an end portion 101a of the cylindrical member 101, since the end portion is more likely to be subjected to heat generated by the conductive ceramic element 104 and heat radiated from the engine. End portions 104a of the conductive ceramic element 104, where the respective electrodes 105 are embedded, are located within the end portion 101a. As shown in FIG. 7B, due to a difference in thermal expansion coefficient between the electrode 105 and the conductive ceramic element 104, a fine defect, such as a gap 105a, may form in the boundary there-between during cooling such as would occur after firing. When the above-mentioned compressive force is exerted on such a defective region, a crack may develop in the conductive ceramic element 104, potentially shortening the life of the conductive ceramic element 104.
At the same time, in order to meet the recent tightening of exhaust gas regulations and to improve fuel consumption ratio, employment of a direct injection system in a diesel engine is rapidly becoming common practice. Thus, there is also a need for increasing the distance between the end of a seat surface and the end of a ceramic heating member by at least 5 mm longer compared with a ceramic heating member used in a swirl-chamber type diesel engine. However, the longer projection of the ceramic heating member into a combustion chamber inevitably leads to thermally induced stress and thus cracking, which may not be sufficiently suppressed simply by disposing within the cylindrical member 101 the boundary between the electrode 105 and the conductive ceramic element 104.